Method for removal of recalcitrant selenium species from wastewater

ABSTRACT

Methods and systems for reducing the concentration of selenium species in water, particularly water containing recalcitrant selenium species. In the methods and systems, water containing one or more selenium species is treated with permanganate to provide permanganate-treated water, which is then contacted with a zero-valent iron treatment system comprising (a) a reactive solid comprising zero-valent iron and one or more iron oxide minerals in contact therewith and (b) ferrous iron.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/244,613, filed Oct. 21, 2015, which application is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to improved methods for removing seleniumspecies from wastewater, particularly removing recalcitrant seleniumspecies using a hybrid zero-valent iron system that includes anoxidation pretreatment stage.

BACKGROUND OF THE INVENTION

Selenium is present in a variety of industry wastewaters and isincreasingly recognized as a pollutant of significant concern. In recentyears, both federal and local environmental regulatory bodies have movedtowards imposing strict limits for selenium concentrations in industrialeffluent discharges. In industrial wastewaters, selenium may be presentin various forms. Selenium oxyanions, such as selenate (SeO₄ ²⁻) andselenite (SeO₃ ²⁻) are more commonly found in coal-fired power plant andmining wastestreams, and reduced forms, such as selenocyanate (SeCN⁻),are often present in the sour-stripped water from the oil refiningprocess.

Removing selenium from wastewater represents a challenge and a mandateto the water industry. In 2013, the US Environmental Protection Agencyproposed a limit of 10 ppb (μg/L) for selenium monthly average for thethermoelectric power industry. Some local and state authorities haveenacted even lower limits for certain wastewater effluents (e.g., 4.7ppb for total selenium imposed in mining and refinery wastewater in somestates). To comply with such strict limits, industries must oftenachieve over 99% removal efficiency.

In view of the requirement for compliance to regulations for effluentdischarge limits, many power plants have installed wet flue gasdesulfurization (FGD) systems to control sulfur dioxide emissions. Inaddition to sulfur dioxide, wet FGD systems also capture volatile traceelements including selenium and these FGD wastewaters can containdissolved selenium in concentrations ranging from less than 10 ppb toseveral thousand ppb.

Selenium exists in a variety of forms and oxidation states. Organicselenium species present in the environment include selenide (Se⁻²),elemental selenium (Se⁰), selenite (Se⁴⁺), and selenate (Se⁶⁺).Elemental selenium (Se⁰) and selenides (Se⁻²) exist in reducing zonesand unweathered mineral formations, and are relatively immobile becauseof the low solubility of their solid phases. Selenite (Se⁴⁺) andselenate (Se⁶⁺) are the most mobile forms of selenium and their primaryspecies at neutral conditions are HSeO₃ ⁻ and SeO₄ ²⁻, respectively. Inmany selenium-contaminated wastewaters, selenium exists in a few commonforms including selenate, selenite, selenocyanate, and methylselenicacid.

Several chemical and biological methods have been demonstrated to beeffective for removing these selenium compounds in wastewater to a verylow concentration level. However, in certain wastewaters, selenium hasbeen found to exist in unidentified forms that are difficult to removeby established selenium treatment methods.

Selenium chemistry in the FGD wastewater is complex due totransformations between selenium species and the presence ofunidentified selenium species that are resistant to removal.

While certain zero-valent iron technologies (e.g., hybrid zero-valentiron) have been shown to efficiently reduce the concentrations ofidentifiable organic and inorganic selenium species, such as selenate,selenite, methylselenic acid, selenomethionine, and selenocyanate, inwastewater to low or sub ppb levels (<10 ppb), removal of unidentifiedselenium species that are resistant to removal remains a challenge.

To date, treatment methods have been less effective in achieving lowselenium concentrations for wastewaters that include unidentifiedselenium species that are resistant to removal. The presence of suchselenium species is an obstacle for industries where regulations areforcing compliance with ever-more stringent effluent discharge limitsfor selenium.

A need exists for improved methods for removing or reducing theconcentration of selenium in industrial wastewaters, particularlywastewaters that include selenium species that have been previouslyresistant to removal. The present invention seeks to fulfill this needand provides further related advantages.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a method for reducing theconcentration of selenium species in water. In one embodiment, themethod comprises:

treating water comprising one or more selenium species with permanganateto provide permanganate-treated water; and

contacting the permanganate-treated water with a zero-valent irontreatment system comprising (a) a reactive solid comprising zero-valentiron and one or more iron oxide minerals in contact therewith and (b)ferrous iron, whereby the concentration of selenium in thepermanganate-treated water is reduced by the action of the zero-valentiron system on the selenium species.

In another aspect, the invention provides a system for reducing theconcentration of selenium species in water. In one embodiment, thesystem comprises:

a first vessel for receiving water comprising one or more seleniumspecies, wherein the first vessel comprises aqueous permanganate; and

a first reactor in fluid communication with the first vessel forreceiving permanganate-treated water from the first vessel, wherein thefirst reactor comprises a zero-valent iron treatment system comprising(a) a reactive solid comprising zero-valent iron and one or more ironoxide minerals in contact therewith and (b) ferrous iron.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings.

FIG. 1 is a schematic illustration of a representative two-stage hybridzero-valent iron (hZVI or activated iron) treatment system: permanganateoxidation pre-treatment (OX PT); first stage zero-valent iron reduction(R1); and second stage zero-valent iron reduction (R2).

FIG. 2 compares selenium species distribution [Se(IV), Se (VI),MeSe(IV), and unidentified Se (unknown Se)] in FGD wastewater as percenttotal selenium of untreated (raw) and pre-treated by select oxidants(hydrogen peroxide (H₂O₂), potassium permanganate (KMnO₄), chlorine(hypochlorite), persulfate).

FIG. 3 illustrates total selenium concentration (ppb) in FGD wastewateras a function of reaction time during a representative batch test.

FIG. 4 compares selenium removal (Se removal, %) for FGD wastewater in arepresentative treatment method (continuous-flow stirred-tank reactortest) of the invention: two-stages with a hydraulic residence time (HRT)of 6 h.

FIG. 5 compares selenium removal (Se removal, %) for FGD wastewater in arepresentative treatment method (continuous-flow stirred-tank reactortest) of the invention: two-stages with a total hydraulic residence time(HRT) of 4 h.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods and systems for reducing theconcentration of selenium species in water, particularly watercontaining recalcitrant selenium species. In the methods and systems,water containing one or more selenium species is treated withpermanganate to provide permanganate-treated water, which is thencontacted with a zero-valent iron treatment system comprising (a) areactive solid comprising zero-valent iron and one or more iron oxideminerals in contact therewith and (b) ferrous iron.

In the methods of the invention, permanganate oxidation occurs prior tothe zero-valent iron treatment (e.g., first stage of a multiple stagezero-valent iron treatment). In certain embodiments, permanganate isadded to the wastewater feed tank to mix with raw wastewater and oxidize(or destroy) the recalcitrant selenium species to provide seleniumspecies treatable by the zero-valent iron system.

Methods for Reducing the Concentration of Selenium Species

In one aspect, the invention provides a method for reducing theconcentration of selenium species in water. In one embodiment, themethod for reducing the concentration of selenium species in water,comprises:

treating water comprising one or more selenium species with permanganateto provide permanganate-treated water; and

contacting the permanganate-treated water with a zero-valent irontreatment system comprising (a) a reactive solid comprising zero-valentiron and one or more iron oxide minerals in contact therewith and (b)ferrous iron, whereby the concentration of selenium in thepermanganate-treated water is reduced by the action of the zero-valentiron system on the selenium species.

The method of the invention is effective in reducing the concentrationof selenium species in water, particularly effective in reducing theconcentration of recalcitrant selenium species in water. As used herein,the term “recalcitrant selenium species” refers to selenium speciesfound in selenium-containing wastewaters that are at present inunidentified forms and that are difficult to remove or to have theirconcentrations reduced by established selenium treatment methods,including hybrid zero-valent iron treatment methods (e.g., ActivatedIron Process).

Unless otherwise stated, selenium concentrations (ppb or g/L) are totalselenium concentrations as determined by inductively coupled plasmadynamic reaction cell mass spectrometry (ICP-DRC-MS).

Selenium speciation was determined by ion chromatography inductivelycoupled plasma collision reaction cell mass spectrometry(IC-ICP-CRC-MS).

In certain embodiments, the concentration of permanganate useful in themethod is from about 1 to about 100 mg/L. In certain embodiments, theconcentration of permanganate is from about 10 to about 50 mg/L. Incertain embodiments, the concentration of permanganate is about 10 mg/L.

The source of permanganate is not critical. In certain embodiments, thepermanganate is potassium permanganate. Other permanganate salts areeffective.

A variety of selenium species are effectively treated by the method ofthe invention. Selenium species that are effectively treated includeselenate (Se⁶⁺), selenite (Se⁴⁺), and selenide (Se⁻²) species, as wellas mixtures thereof. Representative selenium species that areeffectively treated include selenates, selenites, selenocyanates,selenomethionines, and methylselenic acids. As noted above, the methodof the invention is particularly effective in treating (i.e., reducingthe concentration of) recalcitrant selenium species.

In certain embodiments of the method, treating water comprises one ormore selenium species with permanganate to provide permanganate-treatedwater occurs in a first vessel (e.g., contact mixing tank). In thisembodiment, the concentration of permanganate in the first vessel ismaintained at about 5 to about 100 mg/L. In certain embodiments,contacting the permanganate-treated water with a zero-valent irontreatment system occurs in a first reactor. In these embodiments, thefirst reactor is a packed bed or a fluidized bed. In certainembodiments, the ferrous iron of the zero-valent iron treatment systemis continuously introduced to the first reactor.

It will be appreciated that the methods of the invention can includemore than one zero-valent iron reactor (e.g., multi-stage zero-valentiron reactors include in two, three, four, five, six, seven, or eightzero-valent iron reactors).

In certain embodiments, the method does not include introducing air oroxygen (aeration) to the water containing the selenium species either inthe permanganate treatment step or the zero-valent iron treatment step.

The present invention provides an improved method for reducing theconcentration of selenium species in water involving contacting thewater containing the one or more selenium species with a zero-valentiron treatment system comprising (a) a reactive solid comprisingzero-valent iron and one or more iron oxide minerals in contacttherewith, and (b) ferrous iron, the improvement comprising treating thewater comprising one or more selenium species with permanganate prior tocontacting the water with the hybrid zero-valent iron treatment system.

It will be appreciated that the permanganate treatment step of themethod of the invention can be effectively coupled with any treatmentsystem that is effective for removing or reducing the concentration ofcontaminants. The permanganate treatment step can be combined withtreatment systems other than zero-valent iron treatment systems and withzero-valent iron treatment systems that are not hybrid zero-valent ironsystems (i.e., zero-valent iron systems that utilize zero-valent ironalone without the use of ferrous iron to maintain the activity of thezero-valent iron).

Zero-Valent Iron Treatment System.

The methods of the invention utilize a zero-valent iron treatment system(Fe(0)/FeO_(x)/Fe²⁺) for reducing the concentration of the seleniumspecies. In the method, water containing one or more selenium species istreated with permanganate to provide permanganate-treated water, whichis then contacted with a zero-valent iron treatment system comprising(a) a reactive solid comprising zero-valent iron and one or more ironoxide minerals in contact therewith and (b) ferrous iron. In certainembodiments, the one or more iron oxide minerals of the reactive solidcomprise magnetite. In certain embodiments, the reactive solid comprisesa plurality of particles.

Zero-valent iron treatment systems that utilize ZVI composites to reducethe concentration of a variety contaminants are useful in the methods ofthe invention and include those described in US 2011/0174743 and US2012/0273431, each expressly incorporated by reference in its entirety.In these zero-valent iron treatment systems, also known as hybridzero-valent iron (hZVI) treatment systems, a zero-valent iron[Fe(0)/FeO_(x)/Fe²⁺] composite (also referred to as a hybrid zero-valentiron composite or hybrid ZVI composite) includes a reactive solid [zerovalent iron (Fe(0) or ZVI) and iron oxide (FeO_(x))] and a secondaryreagent [ferrous iron, (Fe(II), or Fe²⁺)]. In the methods, the reactivesolid is effective for removing and/or reducing the concentration ofcontaminants in a fluid. In certain embodiments, the composite is aparticle having a core comprising zero-valent iron and a layerassociated with the core that includes the reactive material.

In zero-valent iron treatment systems, the zero-valent iron serves as areductant that is effective to reduce the contaminant species such thatthe reducible contaminant species is removed from solution and convertedinto a solid, thereby effectively reducing the concentration of thecontaminant in solution (e.g., contaminated water). The ferrous iron ofthe system serves to maintain the activity of the zero-valent ironcomponent of the system.

An advantage of the hybrid ZVI composite and system is thesustainability of a high level of activity and improved lifetime,particularly in comparison to compositions or systems that includezero-valent iron alone (i.e., without supplemental ferrous iron).

The reactive composite can be produced by an activation process. Theactivation process may involve oxidizing at least a portion of azero-valent iron so as to form an iron oxide and exposing the iron oxideto dissolved ferrous ion to form the reactive material. The ferrous ionmay adsorb onto and become a part of the composite. The reactivecomposite may be produced in situ as part of a contaminant removalprocess.

Treatable Contaminated Fluids.

A variety of selenium-containing waters may be treated according to themethods of the invention. Representative treatable waters include fluegas desulfurization wastewater, industrial waste streams, oil refinerywaste, tail water of a mining operation, stripped sour water, surfacewater, ground water, and an influent stream. Industrial waste streamsinclude streams of various industrial processes. An industrial wastestream treatable by the method of the invention can be produced at anystage of an industrial process. In one embodiment, the water is a fluegas desulfurization (FGD) wastewater. In one embodiment, the water isoil refinery waste. In one embodiment, the water is tail water of amining operation. In one embodiment, the water is stripped sour water.

Contaminants and Contaminant Removal.

In addition to reducing the concentration of selenium, the methods ofthe invention are effective for reducing the concentration of othercontaminants whose concentration can be reduced by zero-valent ironsystem treatment.

Contaminants that can be removed or their concentration reduced includemetal compounds, metal ions, metal oxides, metalloids, oxyanions,chlorinated organic compounds, and combinations thereof.

Examples of contaminants treatable by the methods of the inventioninclude toxic materials, such as toxic metals. Non-limiting examples oftoxic metals include arsenic, aluminum, antimony, beryllium, mercury,cobalt, lead, cadmium, chromium, silver, zinc, nickel, molybdenum,thallium, vanadium, and the like, ions thereof, and compounds thereof.

Treatable contaminants can include metalloid contaminants, such as boronand ions thereof; oxyanions, such borates, nitrates, bromates, iodates,and periodates; and chlorinated organic compounds.

Waters treatable by the methods of the invention can include mixtures ofthe contaminants noted above.

Representative contaminants that can be removed or their concentrationreduced include arsenic compounds, aluminum compounds, antimonycompounds, beryllium compounds, mercury compounds, cobalt compounds,lead compounds, cadmium compounds, chromium compounds, silver compounds,zinc compounds, nickel compounds, molybdenum compounds, thalliumcompounds, vanadium compounds, arsenic ion, aluminum ion, antimony ion,beryllium ion, mercury ion, selenium ion, cobalt ion, lead ion, cadmiumion, chromium ion, silver ion, zinc ion, nickel ion, molybdenum ion,thallium ion, vanadium ion, borates, nitrates, bromates, iodates,periodates, trichloroethylene, dissolved silica, and combinationsthereof.

Systems for Reducing the Concentration of Selenium Species

In another aspect, the invention provides systems for reducing theconcentration of one or more selenium species in water. In oneembodiment, the system comprises:

a first vessel for receiving water comprising one or more seleniumspecies, wherein the first vessel comprises aqueous permanganate; and

a first reactor in fluid communication with the first vessel forreceiving permanganate-treated water from the first vessel, wherein thefirst reactor comprises a zero-valent iron treatment system comprising(a) a reactive solid comprising zero-valent iron and one or more ironoxide minerals in contact therewith and (b) ferrous iron.

Exemplary zero-valent iron treatment systems useful in the methods ofthe invention include those described in US 2011/01747443 and US2012/027343, each expressly incorporated herein by reference in itsentirety. Single-stage and multiple-stage reactor systems can be used.

In some embodiments, the system is a single-stage reactor system andincludes a single reactor (e.g., a fluidized bed reactor). In otherembodiments, the system is a multiple-stage reactor system and includestwo or more reactors. The systems may further include one or more of thefollowing: an internal solid/liquid separating zone (e.g., settlingzone), an aerating basin, a settling basin, and a filtration bed.

FIG. 1 is a schematic illustration of a representative two-stage hybridzero-valent iron (hZVI or activated iron) treatment system: permanganateoxidation pre-treatment (OX PT); first stage zero-valent iron reduction(R1); and second stage zero-valent iron reduction (R2). Referring toFIG. 1, system 100 includes oxidation pre-treatment (OX PT) vessel 10comprising aqueous permanganate (e.g., permanganate in raw wastewater)which provides oxidant-treated wastewater 15; first hZVI reactor R1 (30a); and second hZVI reactor R2 (30 b). Ferrous iron solution 25 isdelivered from vessel 20 to first reactor 30 a (R1) via pump 22.Oxidant-treated wastewater 15 is delivered to first reactor 30 a viapump 12. First reactor 30 a includes first stage ZVI-treated water 35 a,which is conducted to second reactor 30 b (R1) to provide second stageZVI-treated water 35 b. Each of the first and second reactors includes astirrer (32 a and 32 b, respectively). Treated effluent 40 is conductedfrom second reactor 30 b.

Representative Embodiments

Pilot tests conducted at a Southern Company Inc. power plant (Atlanta,Ga.) on FGD wastewater demonstrated that although a hybrid zero-valentiron (hZVI) system was effective in decreasing concentrations ofselenate, selenite, selenocyanate, and other currently identifiableorganoselenium compounds to low ppb level (<5 ppb), certain unidentifiedselenium species were very difficult to decrease to below 10 ppb. In theFGD wastewater, unidentified selenium species at a concentration of200-700 ppb accounted for about 20% of the total selenium. During thetesting, the unidentified selenium species was generally decreased froma level of a few hundred ppb in the feed to about 50 ppb in the treatedeffluent after treatment with 4-stage hZVI reactor having a 17 hrreaction time. The 17 h result, while not so impressive when comparedwith the removal of more common selenium species (e.g., selenate,selenite, selenocyanate) by the hZVI system, indicate that the hZVIsystem can slowly remove these unidentified selenium species to asignificant degree. Because the FGD wastewater was found to contain asubstantial concentration of organic carbon (TOC=22 mg/L), it waspostulated that the unidentified selenium species may be related toorganic material in the wastewater.

To test the hypothesis, laboratory batch and continuous flowstirred-tank reactor (CSTR) tests were conducted to evaluate theeffectiveness of the hZVI system in removing or reducing theconcentration of these unidentified selenium species present in the FGDwastewater, and to evaluate modifications of the hZVI system to improvethe effectiveness of removal of the unidentified selenium species.

As a result of these studies, a chemical method was developed thatutilizes a strong oxidant, permanganate, to pretreat wastewatercontaminated with these unidentified selenium species. After theoxidation pretreatment, the hZVI system was used to treat thepermanganate-pretreated wastewater to provide wastewater effluents inwhich the selenium concentration was reduced to less than 10 ppb (μg/L).For these wastewaters, without permanganate pretreatment, the hZVIsystem provided wastewater effluents having total selenium concentrationof about 50 ppb.

Without being bound to theory, two mechanisms may be involved in thepermanganate pretreatment: (1) the unidentified selenium species may bedirectly oxidized by permanganate into selenate; or (2) if unidentifiedselenium species existed as part of a greater organic structure (e.g., aprotein structure), permanganate treatment may participate in thebreakdown of the greater organic structure to smaller structures thatcan be effectively treated by the hZVI system or a biological method.

The permanganate pretreatment is not designed to remove selenium orreduce selenium concentration. Rather, permanganate pretreatment iseffective to transform certain unidentified selenium species to seleniumspecies treatable by established selenium-removal systems and methods(e.g., hZVI). The present invention therefore provides a system andmethod for improving the treatability of selenium-containingwastewaters, particularly selenium-containing wastewaters that includeunidentified selenium species, in subsequent wastewater treatments.

Oxidation of FGD Wastewater Samples of FGD wastewater were treated withselect oxidants (i.e., hydrogen peroxide (H₂O₂), potassium permanganate(KMnO₄), hypochlorite (OCl⁻), and persulfate (S₂O₈ ²⁻) at 50 mg/L) todetermine the effectiveness of oxidants to transform the unidentifiedselenium species to identifiable species and to evaluate seleniumspeciation. See Example 1.

Selenium speciation results indicate that selenium in the tested FGDwastewater existed in various species and oxidation states, and that thetotal selenium concentration in the FGD wastewater was about 1788 ppb byIC-ICP-CRC-MS method. Table 1 summarizes selenium speciation change inFGD wastewater after treatment with select oxidants (hydrogen peroxide,potassium permanganate, hypochlorite (OCl⁻), and persulfate (S₂O₈ ²⁻).

TABLE 1 Change of Se speciation in the FGD wastewater after treatmentwith selected oxidants. Se speciation analyses (IC-ICP-CRC-MS) UnknownSe Samples pH Se(IV) Se(VI) SeCN⁻ MeSe(IV) SeMe SeSO₃ ²⁻ Species (n)Total Se Raw FGD 7.54 371 ND¹ 5.03 ND² ND² 52.6 (1) 1788.63 Pretreated7.37 1410 384 ND¹ 11.1 ND² ND² 54.9 (1) 1860.00 with H₂O₂ Pretreated7.31 1.58 1690 ND¹ 21.2 ND² ND²  35 (1) 1747.78 with KMnO₄ Pretreated7.77 2.26 1760 ND¹ 11.2 ND² ND² 57.9 (1) 1831.36 with OCl⁻ Pretreated7.03 1280 505 0.79 10.4 ND² ND² 53.9 (1) 1850.09 with S₂O₈ ²⁻ Allresults reflect the applied dilution and are reported in μg/L. ND¹ = Notdetected at the applied dilution (<0.35). ND² = Not detected at theapplied dilution (<0.30). SeCN = Selenocyanate; MeSe(IV) =Methylseleninic acid; SeMe = Selenomethionin; SeSO₃ ²⁻ = Selenosulfate.Unknown Se Species = Total concentration of all unknown Se speciesobserved by IC-ICP-Ms. n = number of unknown Se species observed.

As shown in FIG. 2, selenite in raw FGD wastewater accounted for 76% ofthe selenium and was the dominant species, while selenate andmethylseleninic acid accounted for 20.7% and 0.28%, respectively. Thedata suggested that a substantial fraction of unidentified selenium(unknown Se) (2.94%) cannot be definitively identified by currentselenium speciation methodology. The addition of hydrogen peroxide(H₂O₂), potassium permanganate (KMnO₄), hypochlorite (OCl⁻), andpersulfate (S₂O₈ ²⁻) was used to determine the effectiveness of oxidantsto transform the unidentified selenium species to simpler,currently-identifiable selenium species, such as selenate or selenite,for efficient removal by zero-valent iron systems and methods (e.g.,hZVI). The original pH of FGD wastewater was 7.54 and the final pHranged from 7.03 to 7.77 after adding different oxidants (Table 1). NopH adjustment was made during the test.

Selenium speciation results (FIG. 2) showed that methaneseleninic acid(MeSe) concentration was increased by all oxidants tested because MeSeis readily synthesized by oxidation. No other obvious change was foundby the addition of hydrogen peroxide suggesting that hydrogen peroxidecannot improve total selenium removal. When persulfate was used asoxidant, selenite decreased from 76% to 69.2%, while selenate increasedfrom 20.7% to 27.3%, which indicated that persulfate can oxidizeselenite to selenate (see FIG. 2). Selenocyanate (SeCN⁻) was slightlyincreased upon persulfate oxidation. Oxidation by potassium permanganateand hypochlorite decreased selenite from 76% to 0.09% and 0.12%,respectively, and increased selenate from 20.7% to 96.7% and 96.1%,respectively. Among the tested oxidants, only potassium permanganate (50mg/L) was effective in reducing unidentified selenium from 52.6 to 35.0ppb.

Batch Test for FGD Wastewater

A batch test was conducted to evaluate the effectiveness of azero-valent iron (i.e., hZVI) system with oxidative pretreatment forselenium removal from FGD wastewater. See Example 2.

A batch hZVI test was conducted in a 6 L reactor with FGD wastewaterpretreated with 100 mg/L KMnO₄. As shown in FIG. 3, total selenium wasrapidly decreased from 1720 μg/L to 51 μg/L in the first 12 h, and thento 4.66 μg/L (total selenium) (greater than 99.7% removal) at 24 h.Zero-valent iron (e.g., hZVI) methods are effective for treating commonidentifiable selenium compounds in FGD wastewater. Therefore, theresidual selenium (4.66 ppb) after treatment was likely the unidentifiedselenium species (such a result meets the selenium discharge limit of 10ppb planned by the US EPA). In a control batch hZVI test treating thesame FGD wastewater but without permanganate pretreatment, totalselenium was reduced to about 50 ppb after 24 h reaction and thereafterthe residual selenium remained steady after 48 h treatment.

As noted above, unidentified selenium in FGD wastewater decreasedmarkedly when pretreated with 50 mg/L KMnO₄ (Table 1). By increasing thepermanganate concentration, it is believed that additional quantities ofunidentified selenium can be broken down to other reactive seleniumspecies that can be effectively removed by zero-valent iron (e.g., hZVI)systems.

The results demonstrate that most unidentified selenium in FGDwastewater can be removed by the hZVI system after pretreatment withpermanganate.

CSTR Test for FGD Wastewater

A two-stage continuously stirred-tank reactor (CSTR) test was conductedto evaluate the effectiveness of a zero-valent iron (i.e., hZVI) systemwith oxidative pretreatment for selenium removal from FGD wastewater.See Example 3 and FIG. 1.

Selenium results for FGD wastewater CSTR tests are summarized in Table2. Table 2 summarizes total selenium quantitation and speciation in FGDwastewater after pretreatment with FGD wastewater after pretreatmentwith 50 ppm KMnO₄ in a representative constant stirred reactor (CSTR)test.

TABLE 2 Total selenium quantitation and speciation in the FGD wastewaterafter pretreatment with 50 ppm KMnO₄ in the CSTR test. Se speciationanalyses (IC-ICP-CRC-MS) Unknown Total Se Fe²⁺ Se Species Total (by ICP-Missing Samples pH ppm Se(IV) Se(VI) SeCN⁻ MeSe(IV) SeMe SeSO₃ ²⁻ (n) SeDRC-MS) Se Raw FGD 7.52 1330 357 ND² 9.38 ND³ ND³ 66.3 (1) 1762.7 2050287.3 Pretreated with KMnO₄, 1 h 7.47 1.8 1670 ND² 22.2 ND³ ND³  35 (1)1729.0 2020 291.0 HRT = 6 h R1 at t = 18 h 8.61 0 ND¹ 87.9 ND² 2.57 ND³ND³ 2.14 (1) 92.61 116 23.39 R2 at t = 18 h 8.80 0 ND¹ 6.26 ND² 1.17 ND³ND³ 0 7.43 9.5 2.07 R1 at t = 24 h 8.58 0 ND¹ 68.8 ND² 2.42 ND³ ND³ 2.17(1) 73.39 87.3 13.91 R2 at t = 24 h 8.81 0 ND¹ 5.29 ND² 0.91 ND³ ND³ 06.20 8.0 1.80 HRT = 4 h R1 at t = 16 h 8.47 1.14 ND¹ 123 ND² 2.57 ND³ND³ 5.31 (1) 130.9 167 36.12 R2 at t = 16 h 8.68 0.615 ND¹ 13.1 ND² 0.84ND³ ND³ 0 13.94 22.7 8.76 R1 at t = 20 h 8.50 0.915 ND¹ 118 ND² 3.07 ND³ND³ 5.16 (1) 126.2 139 12.77 R2 at t = 20 h 8.73 0.48 ND¹ 11.6 ND² 0.74ND³ ND³ 0 12.34 18.7 6.36 All results reflect the applied dilution andare reported in μg/L. ND¹ = Not detected at the applied dilution)<0.74). ND² = Not detected at the applied dilution (<0.16). ND³ = Notdetected at the applied dilution (<0.49). SeCN⁻ = Selenocyanate;MeSe(IV) = Methylseleninic acid; SeMe = Selenomethionine; SeSO₃ ²⁻ =Selenosulfate. Unknown Se Species = Total concentration of all unknownSe species observed by IC-ICP-MS. n = number of unknown Se speciesobserved.

The total selenium concentration after pretreatment was about 1729 and2020 ppb by IC-ICP-CRC-MS and ICP-DRC-MS methods, respectively. Themissing selenium after pretreatment represents about 14.4% of total Se,which was calculated by the differences of total selenium detected bythe two methods. The consistent discrepancy of the two methods indicatedthat there were certain selenium species in the sample that might havebeen filtered out by the IC column and escaped detection byIC-ICP-CRC-MS. Here total selenium was analyzed on the basis ofICP-DRC-MS method.

As shown in Table 2, 50 mg/L KMnO₄ reduced the concentration ofunidentified selenium (unknown Se) from 66.3 to 35.0 ppb. With theaddition of permanganate, selenite was decreased from 64.88% tonegligible (0.09%) and selenate increased accordingly from 17.41% to82.67%, indicating that selenite was completely oxidized to selenate bypermanganate. Methaneseleninic acid (MeSe) was increased from 9.38 to22.2 ppb.

After pretreatment with 50 mg/L KMnO₄ (as Mn) for 1 h, two stage of CSTRtest was conducted to evaluate selenium removal by hZVI. At HRT=6 h,total selenium concentration measured by ICP-DRC-MS in the first stage(R1) after 18 h (three HRT time passed) was reduced from 2020 to 116 ppb(Table 2). Meanwhile, all selenite and 93.89% unidentified selenium wasremoved in R1 and all unidentified selenium was removed by the secondstage (R2), as shown in FIG. 4. Total selenium in R2 was only 9.5 ppb(Table 2), which was composed of 6.26 ppb Se (VI), 1.17 ppb MeSe (IV),and 2.07 ppb missing Se from calculation. Total selenium in R1 and R2after four HRTs was 87.3 and 8.0 ppb, respectively, which represented99.6% of removal efficiency after the two-stage process. When HRT wasadjusted to 4 h, total selenium in R1 and R2 after four HRTs was 167 and22.7 ppb, respectively (Table 2). All selenite was removed in R1 and allunidentified selenium was removed in R2 (FIG. 5), which was similar tothe results at HRT of 6 h. Total selenium in R1 and R2 after five HRTswas 139 and 18.7 ppb, respectively. In addition, R1 showed lower pH thanthat of R2, because acidic Fe²⁺ stock solution (pH 2.4) was added intoR1.

Zero-valent iron (e.g., hZVI) systems are known to decrease selenium inFGD wastewater from over 2000 ppb to around 50 ppb in a four-stageprocess with an HRT of 17 h. In this test, the removal efficiency wassignificantly improved after pretreatment with permanganate (e.g., 50mg/L KMnO₄). It is possible that the further unidentified selenium canbe converted to another treatable unidentified selenium species that canbe effectively removed by a zero-valent iron (e.g., hZVI) system,despite the remaining unidentified selenium concentration as high as35.0 ug/L.

The results demonstrate that the hZVI system (two-stage) was effectivefor treating selenium compounds in FGD wastewater after pretreatmentwith permanganate. In these tests, more than 30 mg/L KMnO₄ was left inthe pretreated FGD wastewater feed indicating that 20 mg/L KMnO₄ can besufficient for certain FGD wastewaters. While single-stage processeswith pretreatment can meet the restrictive selenium discharge limitsplanned by the US EPA, two- or three-stage processes with pretreatmentcan meet these limits with shorter reaction times.

The following examples are provided for the purpose of illustrating, notlimiting, the invention.

EXAMPLES Materials and Methods

Materials.

All chemicals used were of analytical reagent grade. All reagentsolutions were prepared with deoxygenated deionized (DDI) water (E-pure,Barnstead, USA). NaNO₃ (>99%, Alfa Aesar) and FeCl₂.4H₂O (J. T. Baker)were used for zero-valent iron preconditioning. The zero-valent iron(Fe⁰ or ZVI) powder (5 μm) was purchased from a commercial ZVI vendorand the purity was about 98% according to the vendor. 20 mM Fe²⁺ addedwith 4 mM HCl was prepared as Fe²⁺ stock solution (pH about 2.4).Potassium permanganate (KMnO₄) J. T. Baker; hydrogen peroxide (30%) BDH;hypochlorite (5% w/v NaClO solution) J. T. Baker; persulfate (10% w/vsolution of Na₂S₂O₉) Aqua Solutions.

Raw flue-gas-desulfurization (FGD) wastewater was obtained from one ofthe power facilities of Southern Company, Inc., Plant Barry, Bucks, Ala.The raw FGD wastewater was known to contain high concentrations ofunidentified selenium species of about 200-700 mg/L (out of totalselenium of about 1000-2000 mg/L) that cannot be effectively removed byvarious treatment methods including both biological treatment andchemical treatment processes conducted by Southern Company, Inc. Theconcentration of unidentified selenium species in the FGD wastewaterdecreased to below 100 ppb pretesting, perhaps due to decomposition ofunidentified selenium species during the long storage time (about 5months).

hZVI System Preconditioning Method.

A nitrate-Fe²⁺ pretreatment method was used to convert a pure ZVI systeminto the hybrid ZVI (hZVI) system (also referred to herein as anactivated iron system or a hybrid ZVI+Fe₃O₄ system) as described inHuang, Y. H.; Zhang, T. C.; Shea, P. J.; Comfort, S. D., Effects ofOxide Coating and Selected Cations on Nitrate Reduction by Iron Metal.Journal Of Environmental Quality 2003, 32, (4), 1306-1315; and Huang, Y.H.; Tang, C.; Zeng, H., Removing molybdate from water using a hybridizedzero-valent iron/magnetite/Fe(II) treatment system. Chemical EngineeringJournal 2012, 200-202, (0), 257-263, each expressly incorporated hereinby reference in its entirety. For the batch test in 6 L reactor andcontinuous-flow test in 2 L reactor, 14.3 mM NaNO₃ (200 mg/L as N), and10 mM FeCl₂ were added into the reactor containing with 50 g/L ZVI. Thereactors were mixed with overhead electric stirrer overnight to allowZVI/Fe(II)-nitrate reaction. The nitrate is reduced by ZVI withmagnetite as the iron corrosion product following the Eq. (1):

NO₃ ⁻+2.82Fe⁰+0.75Fe²⁺+2.25H₂O→NH₄ ⁺+1.19Fe₃O₄+0.50OH⁻   (1)

Upon preconditioning about 5% of the initial ZVI was consumed resultingin a magnetite concentration of about 5 g/L in the reactor. Unlessotherwise stated, tests were conducted with 5 μm ZVI or 325 mesh ZVI.

Sampling and Analysis Methods Sampling kits were purchased from VWR(0.45 μm filter discs, 60 mL syringes, 60 mL plastic vials. Borosilicateglass bottles (40 mL) for selenium sampling were purchased from AppliedSpeciation and Consulting, LLC (ASC) Bothell, Wash. For each test, twosets of samples were collected and filtered through 0.45 μm filters. Noacid was added at the site of collection. One set of collected watersamples were reserved at <4° C. in a cooler filled with ice and sent toASC for selenium speciation and quantitation within 24 hours. The secondset of samples was analyzed at Texas A&M University.

The samples for selenium speciation were analyzed by ion chromatographyinductively coupled plasma collision reaction cell mass spectrometry(IC-ICP-CRC-MS). The samples for total selenium quantitation wereanalyzed by inductively coupled plasma dynamic reaction cell massspectrometry (ICP-DRC-MS).

Example 1 FGD Wastewater Oxidation

In this example, FGD wastewater was treated with a select oxidant andthe treated FGD wastewaters were analyzed for selenium speciation.

To evaluate the effect of chemical methods on unidentified seleniumspecies decomposition, 50 mg/L of a select oxidant (hydrogen peroxide(H₂O₂), potassium permanganate (KMnO₄), hypochlorite (OCl⁻), andpersulfate (S₂O₈ ²⁻)) was added separately into plastic vials containing60 mL raw FGD wastewater. Samples were taken and filtered after 2 h toevaluate the ability of these oxidants to breakdown unidentifiedselenium species and transform these species to currently identifiableselenium species. Selenium speciation was analyzed for these samples.The results are shown in FIG. 2 and summarized in Table 1.

Example 2 FGD Wastewater Treatment: Batch Tests

In this example, FGD wastewater treatment batch tests using an oxidationpretreatment step are described.

Batch tests were conducted to evaluate the effectiveness of azero-valent iron (hZVI) system for removal of unidentified seleniumspecies from FGD wastewater. The effectiveness of selenium removal wasdetermined as a function of reaction time in 6 L reactor. Raw FGDwastewater was added into the reactor containing freshly-preparedactivated iron media (preconditioned hZVI prepared as described above).After stirring overnight, the media was allowed to settle to the bottomof the reactor and the treated supernatant was removed. The treatment offresh media with a dose of FGD wastewater was aimed to eliminate anypotential bias by the fresh activated iron media on selenium removal.

Subsequently, pretreated FGD wastewater that was subject to treatmentwith 100 mg/L KMnO₄—Mn for 1 h was added into the reactor and mixed withthe activated iron media. Samples were taken at 0, 3, 6, 9, 12 and 24 hfor total selenium quantitation.

The results are shown summarized in FIG. 3.

Example 3 FGD Wastewater Treatment: Continuous Stirred-Tank ReactorTests

In this example, continuous stirred-tank reactor (CSTR) FGD wastewatertreatment tests using an oxidation pretreatment step are described.

CSTR tests were conducted to evaluate the effectiveness of a zero-valentiron (hZVI) system for removal of unidentified selenium species from FGDwastewater. The effectiveness of selenium removal was determined as afunction of reaction time using a two-stage CSTR experimental setup,which consisted of two reactors in series, each with a volume of 2 L(see FIG. 1). Fe²⁺ stock solution was continuously added in stage one(R1) to maintain Fe²⁺ in the influent at a concentration of 1.0 mM.

FGD wastewater was pretreated with 50 mg/L KMnO₄ for 1 h, and then wascontinuously flowed into the two-stage CSTR treatment system. Tests wereconducted at a hydraulic retention time (HRT) of 6 h and then 4 h.Samples from FGD wastewater were taken to analyze for pH, Fe²⁺, seleniumspeciation, and total selenium quantitation.

The results are shown in FIGS. 4 (HRT=6 h) and 5 (HRT=4 h) andsummarized in Table 2.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method for reducingthe concentration of selenium species in water, comprising: treatingwater comprising one or more selenium species with permanganate toprovide permanganate-treated water; and contacting thepermanganate-treated water with a zero-valent iron treatment systemcomprising (a) a reactive solid comprising zero-valent iron and one ormore iron oxide minerals in contact therewith and (b) ferrous iron,whereby the concentration of selenium in the permanganate-treated wateris reduced by the action of the zero-valent iron system on the seleniumspecies.
 2. The method of claim 1, wherein the concentration ofpermanganate is from about 5 to about 100 mg/L.
 3. The method of claim1, wherein the concentration of permanganate is from about 10 to about50 mg/L.
 4. The method of claim 1, wherein the concentration ofpermanganate is about 10 mg/L.
 5. The method of claim 1, wherein thepermanganate is potassium permanganate.
 6. The method of claim 1,wherein the one or more selenium species are selected from the groupconsisting of selenate (Se⁶⁺), selenite (Se⁴⁺), and selenide (Se⁻²)species, and mixtures thereof.
 7. The method of claim 1, wherein the oneor more selenium species are selected from the group consisting of aselenate, a selenite, selenocyanate, selenomethionine, and methylselenicacid.
 8. The method of claim 1, wherein the one or more selenium speciescomprises a recalcitrant selenium species.
 9. The method of claim 1,wherein treating water comprising one or more selenium species withpermanganate to provide permanganate-treated water occurs in a firstvessel.
 10. The method of claim 9, wherein the concentration ofpermanganate in the first vessel is maintained at about 1 to about 100mg/L.
 11. The method of claim 1, wherein contacting thepermanganate-treated water with a zero-valent iron treatment systemoccurs in a first reactor.
 12. The method of claim 11, wherein the firstreactor is a packed bed or a fluidized bed.
 13. The method of claim 11,wherein the ferrous iron is continuously introduced to the firstreactor.
 14. The method of claim 1, wherein the water further comprisesa contaminant selected from arsenic, aluminum, antimony, beryllium,cobalt, lead, cadmium, chromium, silver, zinc, nickel, molybdenum,thallium, vanadium, and ions and oxyanions thereof; borates, nitrates,bromates, iodates, and periodates; trichloroethylene; dissolved silica;and mixtures thereof.
 15. The method of claim 1, wherein the waterfurther comprises an oxyanion, a chlorinated organic compound, ormixtures thereof.
 16. The method of claim 1, wherein the water isselected from flue gas desulfurization wastewater, industrial wastestream, oil refinery waste, tail water of a mining operation, strippedsour water, surface water, ground water, and an influent stream.
 17. Themethod of claim 1, wherein the waste is flue gas desulfurizationwastewater.
 18. The method of claim 1, wherein the one or more ironoxide minerals of the reactive solid comprise magnetite.
 19. The methodof claim 1, wherein the reactive solid comprises a plurality ofparticles.
 20. The method of claim 1, wherein the method does notinclude introducing air or oxygen (aeration) to the water containing theselenium species either in the permanganate treatment step or thezero-valent iron treatment step.
 21. In a method for reducing theconcentration of selenium species in water involving contacting thewater containing the one or more selenium species with a zero-valentiron treatment system comprising (a) a reactive solid comprisingzero-valent iron and one or more iron oxide minerals in contacttherewith and (b) ferrous iron, the improvement comprising treating thewater comprising one or more selenium species with permanganate prior tocontacting the water with the zero-valent iron treatment system.
 22. Asystem for reducing the concentration of one or more selenium species inwater, comprising: a first vessel for receiving water comprising one ormore selenium species, wherein the first vessel comprises aqueouspermanganate; and a first reactor in fluid communication with the firstvessel for receiving permanganate-treated water from the first vessel,wherein the first reactor comprises a zero-valent iron treatment systemcomprising (a) a reactive solid comprising zero-valent iron and one ormore iron oxide minerals in contact therewith and (b) ferrous iron.